fellas mos definitely derives from the Greek word Φιλος(Fel(l)os)=friend,i dont know how it became british slang but this is what it mos definitely is.
Fellas are the ironic/joking imagined judgemental male audience of the insecure toxic male. The males addresses fellas, typically to ask if he should avoid doing somethingabsolutely normal so as not to seem gay.